Hello everyone,I want to develop an application with Arduino One in order to soil moisture sensing using Decagon EC-5, which are relativamene at low cost.what's the problem:those sensors must be supplied with a voltage from 2.5 to 3.5 V, however, being capacitive sensors, I can not powered them continuously!I have to power them for about 10ms (about now I can not remember) and then reading the analog channel.therefore a kind of excitation delay!but Could I set OFF pin voltage 3.3V?

You can use one of the digital pins to switch on/off 5V for 10 ms. as long as it draws no more than ~20 mA it should be no problem. otherwise you should use a power transistor to switch on/off the power supply for the sensor.

with a voltage divider you can make 3.3 volt from any 5 volt digitalPin

You can but this device takes 10mA so powering it from a potential divider is not much of an option given there is only 40mA available from a digital pin. As a rule of thumb you need at least ten times the current down a potential divider as you are going to draw out of it.

It would be best to switch the power to this sensor from the 3V3 output through a PNP transistor acting as a top switch.